Conventional optical systems typically suffer from a limited depth of field in the object space, and a corresponding limited depth of focus in the image space. For example, these limitations exist in many electro-optical imaging systems, such as digital cameras, and forward looking infrared (FLIR) systems.
As one specific example, consider a pre-existing state-of-the-art FLIR imaging device with an uncooled detector array. All targets or other objects located in a range of typically about 70 m to infinity will be seen sharply, but anything closer to the imager will be blurred in images from the detector array. If an object suddenly appears at a distance of, for example, 15 m from the imager, it can be difficult or impossible to identify or even recognize the target. It is possible to attempt to refocus the imager, but this requires time and involves moving parts. More importantly, attempting to refocus the imager in this manner will cause distant objects (for example from 50 m to infinity) to be blurred in images, so much so that it will be very difficult to recognize and/or correctly identify those distant objects.
It would be desirable to have an imager that can produce crisp thermal images of multiple objects simultaneously appearing at different distances from 5 m to infinity. However, as a practical matter, it is not a straightforward task to achieve such large depth of field in a conventional portable imager having an uncooled detector. Theoretically, one possible approach would be stopping down the optics. However, this always involves a significant loss of light, which is problematic in thermal imagers with uncooled detectors (because such detectors require very fast optics to operate properly). In addition, stopping down optics will also involve a loss of resolution (at least in well-corrected optics).
A different approach would be to carry out step-by-step focusing, while recording a separate image at each step, and then fuse the multiple recorded images. However, this is a complex and expensive solution that requires moving parts, and that is relatively slow. In addition, during the image fusing process, sharp images of any given object will overlap with unsharp images of that same object taken at other focal steps, which may reduce contrast, and which may necessitate sophisticated and lengthy image-processing routines. Although pre-existing techniques of the type discussed above have been generally adequate for their intended purpose, they have not been satisfactory in all respects.